The production of oil or gas from a drilled well quite commonly involves bore hole operations carried out by means of a variety of tools lowered to various depths within the bore hole. In many situations where the formation traversed by a bore hole contains a number of petroleum-bearing strata at different depths, it is common practice to insert a number of casing strings into the bore hole and to isolate the strata so as to provide multiple zones of petroleum production. After a plurality of casing strings are installed and cemented, it is often necessary to perforate the strings at various depths in order to effect production from each zone. In order to perforate a string without damaging adjacent strings, information regarding the orientation of the perforator is necessary. In any of a number of other bore hole tool operations, it is also necessary to determine the orientation of the tool when it is positioned at a selected depth. Many such tools are lowered on cables which makes it difficult to predict with any certainty the orientation of the tool from the surface.
Efforts have been made in the past to utilize the earth's magnetic field as the basis for determining an azimuth or direction for a particular tool face once positioned at a depth in a bore hole. Unfortunately, there are too many interfering factors associated with the earth's magnetic field brought about by ferrous formations surrounding the bore hole, ferromagnetic casing strings placed within the bore hole, and electrical/electronic tools that generate electromagnetic fields within the bore hole. Given all of these interference factors, other methods of determining tool orientation have generally been focused on. Included among these are a number of radiation-based orientation devices that require adjacent casing strings to be radioactively tagged in order to be avoided by a perforator tool. In addition, various gyroscopic orientation devices have been devised that attempt to detect changes in the tool's orientation as it is lowered into the bore hole. Each of these devices fails to either provide an accurate azimuth for tool face orientation or achieves an accurate azimuth only at the cost of highly complex and expensive equipment.
U.S. Pat. No. 3,704,749 issued to Estes et al. on Dec. 5, 1972, entitled "Method and Apparatus for Tool Orientation in a Bore Hole" describes a method for introducing an axially symmetrical electromagnetic field within the bore hole and providing at least two receiver coils for measuring the magnetic field at an adjacent location. Electronic devices are provided to convert voltages from the receiver coils to a signal that is received at the surface and forms the basis for calculating an orientation azimuth.
U.S. Pat. No. 3,964,553 issued to Basham et al. on Jun. 22, 1976, entitled "Borehole Tool Orienting Apparatus and Systems" describes the use of a moving permanent magnetic assembly designed to generate a magnetic field about the casing string and borehole, and a number of receiver devices to measure the distorted magnetic field due to the presence of ferrous anamolies. The receiver is rotated to produce an azimuthal scan so that the location of the anamolies can be determined.
The Basham et al. patent describes an orienting device in which motion is imparted to a permanent magnet assembly to generate a moving magnetic field and receiver means that generate signals when the magnetic field is distorted due to the presence of a ferrous anamoly. The receiver means are rotated to produce an azimuthal scan such that signals are induced in the receiver means from which the azimuthal location of the anamoly can be determined.
U.S. Pat. No. 4,410,051 issued to Daniel et al. on Oct. 18, 1983, entitled "System and Apparatus for Orienting a Well Casing Perforating Gun" describes a mechanical assembly whereby a perforating gun is appropriately oriented in what is anticipated to be a slant well. The mechanisms of the Daniel et al. patent operate based upon inertial and gravitational forces as opposed to magnetic or radiation methods.
U.S. Pat. No. 5,582,248 issued to Estes, et al. on Dec. 10, 1996, entitled "Reversal-Resistant Apparatus for Tool Orientation in a Borehole" describes an electromagnetic method for accommodating ferrous non-uniformities in the region of the well bore. The method incorporates a measurement of the distortion of the otherwise axially symmetrical electromagnetic field created by the device as it is lowered into a specific casing. The Estes et al. patent includes a device for orientating a tool, such as perforator, with respect to a ferrous body, such as an adjacent casing string, wherein the orienting device utilizes an excitor coil producing an alternating electromagnetic field and a pair of receiver coils longitudinally spaced from the excitor coils. The position of the receiver coils being such that the voltages induced therein vary differentially with the angle presented by the detected ferrous body by reason of the distortion of the otherwise axially symmetric field.
While the prior art electromagnetic orientation devices, such as those described above, allow orientation of a perforator tool or the like with respect to adjacent tubing casing strings, problems arise when in the proximity of large ferrous masses the actual azimuthal orientation "signal" becomes weak as being overridden by the larger ferrous mass.